1. Field of the Invention
This invention relates to a communication receiver, in particular for receiving signals in which a portion of a transmitted signal forms a guard interval.
2. Description of Related Art
The European DVB-T (Digital Video Broadcasting-Terrestrial) standard ETS 300 744 for digital terrestrial television (DTT) uses Coded Orthogonal Frequency Division Multiplexing (COFDM) of transmitted signals, which are therefore grouped into blocks and frames.
It is necessary to transmit the DTT signals over transmission paths which are of uncertain quality. In particular, the area close to the transmission path may include objects such as tall buildings, which cause reflections. That is, a signal may be received at a receiver twice, once on a direct path from the transmitter, and then, after a short delay, as an indirect reflected path. As is well known, this can cause inter-symbol interference (ISI) in the receiver. To reduce this problem, DVB-T COFDM signals include a cyclic prefix guard interval to each active symbol. Specifically, the end portion of the active symbol is repeated before the current active symbol.
These cyclic signals must be correctly removed before demodulation, or the demodulation performance can be seriously degraded. Before the cyclic signals can be removed, it is necessary to know the duration of the guard interval. This is not a constant value, because the presence of the guard interval reduces the efficiency with which the transmission channel can be used, and so it is advantageous to vary the size of the guard interval to be the minimum allowed by the conditions.
The receiver must therefore include a mechanism for determining the size of the guard interval. The chosen mechanism must be robust enough to deal with noisy signals, and with signals which suffer from multipath and co-channel interference.